This invention is directed to an ultraviolet light stabilized polyarylate containing article comprising an article made from a polyarylate coated with a solution of (a) a substituted benzotriazole and a polymer or (b) a substituted benzotriazole.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane also identified as bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. The polyarylates are high temperature, high performance thermoplastic polymers with a good combination of mechanical properties. Thus, they are used to mold a variety of articles.
However, after exposure to ultraviolet (UV) light, including sunlight, an article formed from a polyarylate resin has a tendency to turn yellow. In many applications, such as glazing, such yellowing is not acceptable. In various other applications where light shades of colors or certain blue colorants are used, a color shift changing with time occurs, often leading to non-uniformity of color throughout the material.
One method to prevent yellowing has been to use a stabilizer. However, a number of requirements are imposed upon potential stabilizers; they must be compatible with the polymer, they should not adversely effect the mechanical properties of the polymer, they should not be washed out by solvents, they must be stable to actinic light radiation, and they must be stable at the processing temperatures of the polymer. Thus, the choice of stabilizers which meet such requirements is often a difficult problem. Many stabilizers which are commercially available, such as benzotriazole derivatives, are not effective in preventing yellowing of articles molded from polyarylate resins when utilized as an additive.
Korshak et al in Synthesis and Properties of Self-Protecting Polyarylates, Journal of Polymer Science, Part A-1, Vol. 7, pages 157 to 172 (1969) describe a mechanism to explain the color shift of polyarylates when exposed to UV light. The authors propose that polyarylates undergo the Photo-Fries rearrangement resulting in carbonyl and hydroxyl groups ortho to one another, structurally similar to o-hydroxybenzophenones which are known light absorbers. The Photo-Fries rearrangement leads to a color shift (yellowing) on the surface, perhaps to a depth of approximately 0.5 mils. Thus, the polyarylates have increased photostability in the bulk of the material, but the problem of preventing such surface yellowing exists.
In the present invention it has been found that yellowing of an article made from a polyarylate resin can be substantially reduced by coating the surface of the article with (a) a substituted benzotriazole and a polymer or (b) a substituted benzotriazole, normally contained in a volatile organic solvent. This procedure works surprisingly well even though incorporation as an additive is unsuccessful.